I've worked in a team once here for a jam and other times with friends for some other jams. Everything went well, because we stated what we expected from each other from the beginning. But I know not everyone is so reliable over here.
I work also with a team in my job. We knew each other because we studied together but we weren't friends (at first, now it's another story). Our team suffered a lot of changes. Some people were dissapointing and some people gave more than everyone expected. Fact is, we didn't hesitated to left outside the team anyone who were not involved enough in the work or the people who keep on missing deadlines without a proper reason. You have to be able to say "you are out of the team" without regret if things go wrong. It sounds awful and it's not fun, but sometimes things are like that
And, important advice. Make all of the team members sign a NDA contract, even if just one person, or even if there is not finacial benefitc from the game. Just, do it, it's very important because if things go really wrong, you can take legal actions.
There's A LOT of other factors to take into account about team work, but I'll stop with the rambling